1. Field of the Invention
This invention relates to a method and apparatus for removal at high temperatures of undesirable components, or impurities, from a gaseous stream containing said undesirable components. Depending upon the source of the gaseous stream, e.g. fossil fuel combustion or reforming, the undesirable components that may be addressed by various embodiments of the method and apparatus of this invention include, but are not limited to, NOx, SOx, HgO, H2S, CO2, HCl, and NH3. More particularly, this invention relates to the use of polyoxometalate materials for purification of gaseous streams, such as hydrogen-containing gaseous streams produced by fossil fuel reforming, flue gases produced by fossil fuel combustion, and solid fuel gasification products, and for processing gaseous streams such as natural gas processing, comprising these undesirable components at high temperatures. The polyoxometalate materials selectively remove the undesirable components by absorption and/or diffusion through a layer of the polyoxometalate materials by concentration difference and concentrate them for more effective and efficient removal by currently available technologies.
2. Description of Related Art
H2S, CO2, HCl, and NH3 are byproducts from natural gas, coal gasification or fossil oil reforming which can produce a hydrogen-rich fuel. Removing these contaminants from the hydrogen-rich fuel supplied to proton exchange membrane fuel cell systems (PEMFC) is necessary as the H2S, HCl, and NH3 poison the fuel cell membrane and catalysts. The CO2 and extra water then dilute the fuel and reduce the fuel cell performance. However, these can be removed by other means.
Current techniques for removing these contaminants include low-temperature membrane gas separation to remove CO2 and NH3 at temperatures less than about 120° C., CuO/ZnO catalysts to remove H2S at moderately high temperatures, and Pd-based membranes for hydrogen separation. However, these techniques generally suffer from various limitations including short lifetimes and non-continuous removal of impurities, and they require substantial efforts for regeneration.
Polyoxometalate-based organic-inorganic hybrid materials, which are well-defined, discrete transition metal oxide clusters with a variety of organic ligands as charge-compensating cations, have been applied in many fields, such as catalysis, medicine, materials, surface chemistry, and photo- and electro-chromism. These unique materials are thermally stable at temperatures greater than 300° C. and capable of reversible sorption of gases and organic vapors (CO2, CHCl3, etc.). In addition, polyoxometalates are based on very low-cost starting materials, thereby providing the potential for very attractive manufacturing costs.
Pressure swing adsorption (PSA) is an adiabatic process for purification of gases in which the impurities in the gases are removed by adsorption through suitable adsorbents in fixed beds contained in pressure vessels under high pressure. Regeneration of the adsorbents is accomplished by countercurrent depressurization and by purging at low pressure with previously recovered substantially product-quality gas. To obtain a continuous flow of product, a minimum of two adsorbers is required. In this manner, one adsorber receives feed gas and actually produces a product gas of desired purity while the other adsorber performs the steps of depressurization, purging and repressurization back to the adsorption pressure. After such adsorbent regeneration and repressurization, the functions of the adsorbers are switched. Depending upon the type of impurity to be adsorbed and removed, adsorbents to be used comprise zeolitic molecular sieves, activated carbon, silica gel and activated alumina. Typically, layers of different adsorbent beds are used, thereby dividing the adsorber contents into a number of distinct zones. Monitoring and proper control of process parameters ensures a stable operation. Stable operation means a pendulating swing in each particular location, in adsorber bed or piping, of values for all parameters, i.e. pressure, temperature, flow and composition of gaseous and adsorbed phase.